1. Field of the Invention
This invention relates to a semiconductor laser used in optical signal processing or optical communications, and a method of fabricating the semiconductor laser.
2. Description of the Prior Art
FIG. 7 cross-sectionally illustrates the structure of a conventional semiconductor laser. Reference numeral 701 denotes a GaInP active layer, from which laser light of about 670 nm in wavelength is emitted. Reference numerals 702 and 703 each denote an AlGaInP cladding layer, having a function of confining the laser light to the GaInP active layer 701 and guiding it to an emitting facet. A structure having such a function is called a waveguide. These layers have a larger band gap width than the active layer, and hence at the same time have a function of confining injected carriers in the active layer. Reference numeral 704 denotes an n-type GaAs current blocking layer, which have two functions of preventing currents from passing through this layer and also absorbing the laser light, and, as a result, have a function of confining the laser light to the area beneath a ridge 709 to let the light guide therethrough. Namely, these form a waveguide structure which guides the laser light in the lateral direction. Reference numeral 705 denotes a p-type GaAs contact layer, which is in low-resistivity ohmic contact with a p-type electrode 707. A semiconductor multi-layer structure, comprised of these layers except the electrode 707, is formed on an n-type GaAs substrate 706 having the (100) plane on its surface. Reference numeral 708 denotes an n-type electrode, which is in low-resistivity ohmic contact with the n-type GaAs substrate 706 (see, for example, KOGAKU (Optics), Vol. 19, pp.362-368, 1990).
Of these layers, the GaInP active layer 701 and the p-type and n-type AlGaInP cladding layers 702 and 703 are formed by first crystal growth, the n-type GaAs current blocking layer 704 by second crystal growth, and the p-type GaAs contact layer by third crystal growth.
In this device, the threshold current is 50 mA and the current needed to give a light output of 4 mW is 60 mA, at 25.degree. C.
Fabricating the structure in which the laser light and current are laterally confined in this way brings about a uniform gain, and hence it can be expected that the wavefront of the laser light becomes flat and the astigmatism thereof becomes small. The gain is meant to be the degree to which the intensity of laser light is amplified when the laser light is guided. The wavefront of the laser light swiftly advances at the part where the gain is large. If the astigmatism is large, the spot formed when the light is focused can not be round, so that such laser light can not be readily used as a light source for optical disks.
In such a conventional structure, however, the ridge is in the form of a trapezoid wherein the upside is short, and hence electric currents are laterally spread while they flow from the upside to the active layer 701. As a result, the gain in the active layer 701 becomes gradually smaller toward the base end of the ridge 709. Hence, the wavefront of the laser light swiftly advances at the middle of the ridge, where the astigmatism is enlarged. Thus such laser light can not be readily used as a light source for optical disks.
The n-type GaAs current blocking layer 704 absorbs the laser light and hence has a large guiding loss. The guiding loss is meant to be a loss the laser light may undergo because of absorption or scattering during its passing through the waveguide. This results in an increase in threshold currents or operation currents.
The p-type AlGaInP cladding layer 702 is so high in both resistivity and thermal resistivity that it has been difficult to attain operation at high temperatures or operation in a low droop. The droop is meant to be a gradual decrease in light intensity in one pulse amplitude that may be caused by a decrease in emittion efficiency due to heat generation, at the time of pulsed operation.
The prior art structure also requires carrying out crystal growth three times. In the course of such crystal growth, the structure is heated to a high temperature of 600.degree. C. to 700.degree. C., so that impurities such as zinc present in crystals may move outside the layer because of their diffusion or the like. This may cause an increase in resistivity of the device or result in a poor efficiency for the confinement of carriers to the active layer, tending to bring about a deterioration of characteristics such as threshold currents, high-temperature operation and low-droop operation.